1. Field of the Invention
The present invention relates to red-emitting organic electroluminescent materials and devices in general and in particular to compositions and devices comprising host materials doped with pentacene derivatives.
2. Description of the Related Art
Organic electroluminescent devices have been the subject of considerable research because of their potential applications in a wide variety of flat panel displays, especially ultra thin flat panel displays. Organic electroluminescent devices are very competitive with liquid crystal displays because of their very bright self-emission, low power consumption, low cost of organic materials, ease of color tunability and processability. The present technology will be competing with liquid crystal displays, which are replacing cathode ray tubes as a means of displaying visual information.
One effective approach for achieving color tunability in OLEDs, as well as improving device efficiency and durability, is by doping of fluorescent materials into organic host materials. Organic electroluminescent devices that include organic host materials and dopants are disclosed, for example, in the following patents and publications incorporated herein by reference: U.S. Pat. No. 3,172,862 to Gurnee et al; U.S. Pat. No. 3,173,050 to Gurnee; U.S. Pat. No. 3,710,167 to Dresner et al; U.S. Pat. No. 4,356,429 to Tang; U.S. Pat. No. 4,769,292 to Tang et al; U.S. Pat. No. 5,059,863 to Toshiro et al; U.S. Pat. No. 5,126,214 to Tokailin et al; U.S. Pat. No. 5,382,477 to Saito et al; U.S. Pat. No. 5,409,783 to Tang et al; U.S. Pat. No. 5,554,450 to Shi et al; U.S. Pat. No.5,635,307 to Takeuchi et al; U.S. Pat. No. 5,674,597 to Fujii et al; U.S. Pat. No. 5,709,959 to Adachi et al; U.S. Pat. No. 5,747,183 to Shi et al; U.S. Pat. No. 5,756,224 to Bxc3x6rner et al; U.S. Pat. No.5,861,219 to Thompson et al; U.S. Pat. No. 5,908,581 to Chen et al; U.S. Pat. No. 5,932,363 to Hu et al; U.S. Pat. No. 5,935,720 to Chen et al; U.S. Pat. No. 5,935,721 to Shi et al; U.S. Pat. No.5,948,941 to Tamano et al; U.S. Pat. No. 5,989,737 to Xie et al; International Publication No. WO 98/06242 (Forrest et al); C. W. Tang et al xe2x80x9cElectroluminescence of Doped Organic Thin Filmsxe2x80x9d, J. Appl. Phys. 65(9), May 1969, pp 3610-3616; C. W. Tang and S. A. VanSlyke, xe2x80x9cOrganic Electroluminescent Diodesxe2x80x9d, Appl. Phys. Lett. 51(12), Sep. 21, 1987, pp. 913-915; C. W. Tang, xe2x80x9cOrganic Electroluminescent Materials and Devicesxe2x80x9d Information Display, Oct. 1996, pp. 16-19; J. Shi and C. W. Tang, xe2x80x9cDoped Organic Electroluminescent Devices with Improved Stabilityxe2x80x9d, Appl. Phys. Lett 70(13) Mar. 31, 1997, pp. 1665-1667; Shoustikov et al, xe2x80x9cElectroluminescence Color Tuning by Dye Doping in Organic Light-Emitting Diodesxe2x80x9d, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, No. 1 Jan./Feb. 1998, pp 3-13; Baldo et al, xe2x80x9cHighly Efficient Phosphorescent Emission from Organic Electroluminescent Devicesxe2x80x9d, Nature, Vol.395, Sep. 10, 1998, pp 151xe2x80x94153; O""Brien et al xe2x80x9cImproved Energy Transfer in Electrophosphorescent Devicesxe2x80x9d, Applied Physics Letters, Vol. 74, No. 3, Jan. 18, 1999, pp. 442-444.
Full color electroluminescent displays require efficient red, green and blue electroluminescent materials. Currently, there is a need for more efficient red emitting materials, particularly for compounds with excellent color purity, chemical stability, including photochemical stability and electrochemical stability, high photoluminescence and electroluminescence quantum efficiencies and an extended device lifetime. U.S. Pat. No. 4,769,292 to Tang et al, U.S. Pat. No.5,908,581 to Chen et al; and U.S. Pat. No.5,935,720 to Chen et al. describe compounds such as the DCM/DCJ class of red emitters (4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)4H-pyran and julolidyl derivatives). These compounds typically exhibit broad emission spectra that reduce the red color purity at lower red wavelengths, giving them an orange hue. At higher red wavelengths, the broad emission band results in reduced luminance, since much of the emission is in the long wavelength region where the eye is less sensitive. Moreover, the synthesis of DCM/DCJ compounds results in a by-product that quenches fluorescence. The by-product is difficult to remove during purification and its presence reduces the electroluminescent efficiency of devices using the compounds. U.S. Pat. No. 5,409,783 to Tang et al describes a phthalocyanine compound that has a peak emission in the deep red wavelength region of 660 nm-780 nm. This compound, while useful for photographic printing processes, is less useful in display applications, since the eye has a low response to emission in these wavelengths.
It has now been discovered that pentacene derivatives exhibit very narrow emission spectra and produce a very pure red color in a region of the spectrum that is useful for display applications. When pentacene derivatives are doped into the active emissive layer of organic light-emitting devices, efficient energy transfer from the host material to the pentacene derivatives and/or carrier recombination on the pentacene derivatives takes place, resulting in red electroluminescence predominantly from the pentacene derivatives.
Accordingly, the present invention is directed to an electroluninescent composition comprising a host material and a red dopant, wherein the red dopant is a pentacene derivative substituted with two or more aromatic, substituted aromatic, heteroaromatic or substituted heteroaromatic groups.
Further, the present invention is directed to a heterostructured organic light emitting device for producing electroluminescence, the heterostructure having an emissive layer comprised of a host material and a red dopant, wherein the red dopant is a pentacene derivative substituted with two or more aromatic, substituted aromatic, heteroaromatic or substituted heteroaromatic groups.
The pentacene derivatives of the present invention have the advantages that they are relatively easy to synthesize and that the synthesis process does not produce a by-product that quenches fluorescence (which is a recurring problem in some fluorescent red dyes such as DCM/DCJ compounds). Moreover, the pentacene derivatives have narrow emission spectra in the red visible spectral region appropriate for display applications.